DMR (Digital Mobile Radio) standard is an European standard issued by ETSI (European Telecommunications Standards Institute) for replacing analog PMR (Private Mobile Radio). It has various advantages, such as large coverage area, high transmission rate, high frequency spectrum efficiency and good energy saving effect. As a result, mobile communication products based on DMR standard has a good market prospect.
DMR standard employs a TDMA frame structure with two time slots. FIG. 1 shows a DMR TDMA frame in a direct mode. In the direct mode, communication terminals communicate with each other directly without transfer stations. As shown in FIG. 1, a DMR TDMA frame includes two time slots that are completely the same, wherein the length of each time slot is 30 ms, and it is further divided into two loads with the same length via a centered synchronization pattern field. A protection interval of 1.25 ms is provided at two ends of each time slot, thus an interval of 2.5 ms exists between the two time slots.
However, because in DMR standard, it is not further defined how two time slots in a frame are used to simultaneously support two independent voice calls in the direct communication mode (shorted as “direct mode” below), the existing private network communication terminals based on DMR standard can only carry out a voice call by using one time slot in a frame. FIG. 2 is a schematic diagram showing an application environment 200 of a private network communication terminal based on DMR standard. As shown in FIG. 2, there are a plurality of private network communication terminals 202˜208 in the application environment 200. The private network communication terminal 202 is carrying out a voice call with the private network communication terminal 204. Although the voice call between the private network communication terminal 202 and the private network communication terminal 204 only occupies one time slot of a DMR TDMA frame, the private network communication terminal 206 and the private network communication terminal 208 in the same geographic area cannot carry out a voice call with the other time slot.
The reason for above problem lies in that, although the voice call between the private network communication terminal 202 and the private network communication terminal 204 only occupies one time slot, other private network communication terminals in the same geographic area cannot determine in advance which time slot is an unoccupied time slot because the location of the occupied time slot is not fixed (that is, the occupied time slot may be the first time slot in the DMR TDMA frame or the last time slot in the DMR TDMA frame and the occupied time slot is selected and used by the responding party of the voice call at random). Thus, no new voice calls can be initiated.
Therefore, a technical solution that can overcome the defects in the prior art is needed.